This invention relates to positive displacement pumps, and more particularly to a positive displacement pump which is high in durability being operable continuously for a long period of time, and high in operating reliability.
A root type, claw (phonetic) type, and screw type vacuum pumps are known as positive displacement pumps, and especially as vacuum pumps in the art which are used for evacuation to obtain a low pressure work space. The positive displacement pump must be high in reliability and in durability, being durable in a continuous operation for a long period of time.
A positive displacement pump of this type has been disclosed, for instance, by Japanese Patent Appl cation (OPI) No. 65087/1986 (the term "OPI" as used herein me ns an "unexamined published application"). In the pump, a multi-male thread type rotor, and a multi-female type rotor (which is opposite in the direction of thread) are arranged in parallel with each other in a pump housing, in such a manner that a plurality of shiftable spiral operating chambers are formed between the pump housing and the two rotors. When the two rotors are turned in the opposite directions on the side of one end of the pump (as viewed in the direction of axis of the rotor) which corresponds to the shift start position, the volume communicated with the suction inlet increases, thus performing a gas suction. As a result, the operations chambers having a predetermined volume which have achieved the gas suction are shifted one after another to the side of the other end as viewed in the direction of axis. On the other hand, on the side of the other end (as viewed in the direction of axis of the rotor) which corresponds to the shift end position, the volume which is communicated with discharge outlet decreases, so that the gas in the operating chambers is discharged. The vacuum pump is employed as a multi-stage pump in a semiconductor manufacturing apparatus. In this case, the pump must be high in reliability and in durability for a continuous operation for a long period of time discharging a reactive gas from the semiconductor manufacturing apparatus.
In the above-described positive displacement pump, the pressure of the fluid in the operating chambers increases on the discharge side. Hence, the discharge side becomes high in temperature, while the temperature of the gas suction side is not increased so much. Therefore, the pump suffers from the following difficulties:
In a semiconductor manufacturing process; more specifically, in a process of forming a film by CVD (chemical vapor deposition) for instance, several kinds of gases are supplied to a processing chamber to form the film by chemical reaction. When, during this treatment, the reaction gases are discharged from the treatment chamber with the vacuum pump, solid products stick and deposit on the side of low temperature of the vacuum pump; that is, on the suction side, which makes it difficult for the pump to operate satisfactorily. That is, although the vacuum pump must be so high both in durability and in reliability to the extent that it is operable for a long period of time such as for instance at least one year, it is necessary to overhaul the pump every several months. That is, the conventional vacuum pump is not high enough both in durability and in reliability.